To develop therapeutic and vaccine strategies to combat HIV-1, the cause of AIDS, it is essential to understand the cell-mediated immune response to this agent. To pursue this goal, murine model systems for immunization, isolation of antigen-specific T cells, and in vitro analysis of T cells specific for HIV-1 proteins have been devised. Structure- function studies on a class II-restricted determinant have revealed how a small subset of residues controls binding to MHC molecules, how additional residues regulate the epitopic structure necessary for T cell recognition, and how it may be possible to improve the potency of vaccine antigens by removal of interfering groups in the natural antigen. In particular, an HIV-1 gp160-derived peptide termed T1 that is recognized by both mouse and human T cells together with class II MHC molecules was analyzed. A small subset of residues was found to be sufficient for binding to class II molecules, although not adequate for T cell recognition. Removal of a glutamic acid residue resulted in a 100 fold increase in the antigenic potency of the peptide assayed with a specific T cell clone, and this altered peptide was at least 10 times more effective on a mass basis in inducing responses in mice. This increased potency resulted from an improvement in the MHC class II binding properties of the peptide. These results demonstrate the critical role played in antigen activity by structural features that inhibit rather than contribute to interaction with either MHC molecules or T cell receptors. Recognition of this effect will allow improvement of vaccine constituents and more effective identification of immunogenic regions of proteins when analysis for the absence of interfering groups is included with scans for motifs necessary for promoting MHC molecule binding.